1. Field of the Invention
This invention relates to low noise solid-state preamplifier circuits and, more particularly, to a two-stage integrated circuit amplifier having a three-phase double auto-zeroing switching sequence which enables cancellation of offsets, charge injection, kT/C noise and low frequency flicker noise. The circuit, which is particularly useful in computerized tomography applications, also moderates thermal noise by employing a filtering method that is merged with the overall frequency compensation of the preamplifier.
2. Description of the Prior Art
In computerized tomography (CT), X-ray signals are detected using an array of photodiodes mounted directly behind scintillators. The resolvable dynamic range of the signal to be detected is on the order of one million to one, i.e., 120 dB, due to the range of X-ray attenuation encountered in the human body. Economic and system architectural advantages arise from the ability to completely integrate the preamplifier and subsequent analog-to-digital converter (ADC) together for several CT channels on a single complementary metal oxide semiconductor (CMOS) monolithic integrated circuit (IC). However, the noise properties and power supply levels of CMOS have heretofore limited the dynamic range of any signal conditioning function to roughly 95 dB. Therefore, it would be desirable to achieve 120 dB dynamic range and corresponding linearity in a CMOS photodetector preamplifier circuit.
Prior implementations of CT preamplifiers have utilized discrete components, including low-noise junction field effect transistor (JFET) input bipolar operational amplifiers, to achieve the desirable wide dynamic range. While a low-noise fully integrated CMOS preamplifier for CT has not before been implemented, some CMOS instrumentation amplifiers display certain similarities thereto; however, their basic functions are different. Examples of these CMOS instrumentation amplifiers are presented, for example, by R. D. Yen and P. R. Gray in "A MOS Switched-Capacitor Instrumentation Amplifier", IEEE J. Solid-State Circuits. Vol. SC-17, December 1982, pp. 1008-1013; D. J. Allstot in "A Precision Variable-Supply CMOS Comparator", IEEE J. Solid-State Circuits, Vol. SC-17, December 1982, pp. 1080-1087; B. S. Song and P. R. Gray in "A Precision Curvature-Compensated CMOS Bandgap Reference", IEEE J. Solid-State Circuits, Vol. SC-18, December 1983, pp. 634-643; and B. S. Song, M. F. Tompsett and K. R. Lakshmikumar in "A 12-Bit 1-Msample/s Capacitor Error-Averaging Pipelined A/D Convertor", IEEE J. Solid-State Circuits, Vol. 23, December 1988, pp. 1324-1333. The circuits disclosed in these journal articles address amplification of voltage-mode signals, whereas a photodetector preamplifier must amplify current-mode signals.